In the fabrication of semiconductor devices, lithography is commonly used to form and pattern various layers of a device. Many lithography techniques, particularly photolithography techniques, have become standard practice in the semiconductor industry. For instance, photolithography is used to mask portions of a layer of a device such that only selected portions of the device are either doped or etched. A common photolithographic operation sequence includes depositing a layer of photoresist onto a layer of a semiconductor device before exposing selected portions of the photoresist layer to radiation. The photoresist layer is then developed in a developer solution which either removes portions of the photoresist layer which were removes exposed to radiation in the case of using a positive photoresist or portions of the photoresist layer which were not exposed to radiation in the case of using a negative photoresist. The semiconductor device is then rinsed, typically in de-ionized water (DI water), to completely remove the developer solution from the semiconductor device. The de-ionized water rinse is most often performed in one of two ways, either an immersion rinse in which the device is immersed in DI water or a spray rinse in which the device is sprayed with DI water. After rinsing, the semiconductor device is dried and the subsequent fabrication steps, such as ion implantation, metal etch, sidewall spacer etch, and the like, are completed and the photoresist layer is removed. Photoresist is usually removed either by immersion in an organic solution which dissolves or strips the resist from the device or by ashing the photoresist from the device using a plasma asher. Photoresist may also be removed using various acid solutions provided the semiconductor device has no exposed metal.
A problem has arisen in using common photolithography techniques for defining or patterning certain metal layers on semiconductor devices, particularly metal layers which contain copper. Copper is frequently alloyed with aluminum to reduce hillock formation and to improve aluminum's resistance to electromigration. However, the presence of copper in a metal layer creates a problem of metal microcorrosion during certain photolithography operations. The microcorrosion, also known as pitting, is caused by a galvanic reaction between copper precipitates, namely Al.sub.2 Cu precipitates, near the metal surface and aluminum surrounding the precipitates. Initially, it was believed that the developing solution used to develop the photoresist or the organic solution used to strip the photoresist from the device caused microcorrosion. However, experiments have shown that it is not the developing solution or the organic strip solution, but rather the DI water rinse which follows developing and photoresist stripping, that induces metal microcorrosion. One possible result of such metal microcorrosion is that voids form in metal lines, thereby making a metal line discontinuous and unable to carry a signal. Another possible result is that two lines may be shorted together due to an inadequate metal etch. As the metal corrodes, aluminum oxide forms near the copper precipitate. The aluminum oxide acts as an etch mask during subsequent etching operations such that unwanted metal remains on the device surface, potentially short-circuiting adjacent metal lines.
Because it is essential that semiconductor devices are rinsed following immersion in a developing solution or in an organic photoresist strip solution to prevent contamination of the semiconductor device, some solutions to the problem of microcorrosion are known to have been implemented, none of which completely eliminate the problem. One solution is to make the amount of time a semiconductor device is immersed in the rinse water as short as possible. The less time the device is in DI water, the lower the degree of microcorrosion. However, shortening the time a device is immersed in the rinse water also reduces the adequacy of the rinse. Another potential solution is to rinse the semiconductor device by spraying DI water over the device. By spraying rather than immersing, the amount of time any one area of the device is surrounded by water is reduced and microcorrosion is minimized. However, spray rinsing is not suitable for use in certain applications, for instance in rinsing wafers following an immersion photoresist develop step. Using a spray rinse, the wafers are individually and sequentially rinsed after immersion develop such that the time a wafer is subjected to the developer solution varies depending on the point at which that particular wafer is rinsed. Thus, spray rinsing after an immersion develop operation would result in non-uniform developing of the photoresist, therefore a batch rinse is preferred. In a batch rinse, all wafers are subjected to the developer solution for the same period of time and then simultaneously rinsed, thereby all wafers are developed uniformly. Spray rinsing has other disadvantages. Spray rinsing reduces throughput during fabrication and equipment used in spray rinsing is more expensive than batch rinsing equipment. Furthermore, as semiconductor wafer size increases, batch rinsing will be favored since high pressure spraying can cause substantial damage to larger, more fragile wafers.
Therefore, a need exists for an improved semiconductor fabrication process, and more specifically a process for rinsing semiconductor devices having a metal layer which minimizes microcorrosion of the metal layer, which is easily implemented with existing immersion-type rinse equipment, and which does not increase fabrication cost or number of fabrication steps.